Screening apparatus of the above type is used during both coarse and fine screening of pulp suspensions, preferably for fractionating or separating impurities and other foreign matter which is not desired to be included in the final product, such as shives, knots, coarse particles, scrap, stones or incompletely digested or unrefined chip pieces. The screening apparatus is usually pressurized.
The pulp suspension to be screened is introduced by means of an inlet to the screen chamber where the accepted fraction, the accept, flows through the rotating screen means and into an accept chamber. The accept is then guided down into a lower accept chamber and out through the accept outlet.
The lower accept chamber has the same outer diameter as the screen chamber and is located below the same.
The pulp suspension portion, which does not pass through the screen means (the reject) is discharged by means of the reject outlet, which is usually provided as a tangential outlet in the lower portion of the screen chamber.
Upon rotation of the screen means, a certain pumping effect is obtained in the gap between the knob ring and slot ring. This can result in the accept passing through the gap and out into the screen chamber, thus, preventing the pulp suspension from the screen chamber passing through the gap. In order to increase the pumping effect in the gap, the knob ring and slot ring can be given a greater extension in the radial direction.
The slot ring constitutes one unit with the bottom portion.
When the screening apparatus is in operation, the knob ring and slot ring are subjected to wear. In this manner, the gap increases in size, which results in greater amounts of accept passing through the gap, at the same time as the pumping effect deteriorates. As a consequence, pulp suspension from the screen chamber can pass through the gap. The gap preferably is not wider than the greatest opening in the screen means.
For controlling the gap width, the screening apparatus is stopped, emptied and scavenged. Thereafter, the screening apparatus is dismantled in order to make the gap width accessible to be measured. If the gap width is too great, it can be adjusted either by exchanging the entire screen means or by repairing the knob ring and/or slot ring in such a way that its original thickness is restored.
Measuring of the gap width is very tedious and troublesome. In cases when the gap is found to be too great, high costs will also be involved for repairing, and alternatively exchanging parts, which requires additional time. Due to the fact that it is so troublesome and expensive to change the gap width, the screen is kept in operation far too long, even if the gap width is greater than desirable.